This is not true because you are imagining a world with strict parsing but where people are still acting as though they have lax parsing. In reality, strict parsing changes the incentives and thus people’s behaviour.
This is really easy to demonstrate: we already have a world with strict parsing for everything else. If you make syntax error with JSON, it stops dead. How often is it that you run into a website that fails to load because there is a syntax error in JSON? It’s super rare, right? Why is that? It’s because syntax errors are fatal errors. This means that when developing the site, if the developer makes a syntax error in JSON, they are confronted with it immediately. It won’t even load in their development environment. They can’t run the code and the new change can’t be worked on until the syntax error is resolved, so they do that.
In your hypothetical world, they are making that syntax error… and just deploying it anyway. This makes no sense. You changed the initial condition, but you failed to account for everything that changes downstream of that. If syntax errors are fatal errors, you would expect to see far, far fewer syntax errors because it would be way more difficult for a bug like that to be put into production.
We have strict syntax almost everywhere. How often do you see a Python syntax error in the backend code? How often do you run across an SVG that fails to load because of a syntax error? HTML is the odd one out here, and it’s very clear that Postel was wrong:
